Superconducting tunnel elements detect electromagnetic radiation in a manner that is more sensitive by several orders of magnitude than conventional detectors. Another important advantage is the extremely short response time of superconducting tunnel elements, which is currently in the range of picoseconds (tmin.gtoreq.e.multidot..PHI..sub.0 /DELTA; wherein e=electron charge; .PHI..sub.0 =flux quantum; and DELTA=energy gap).
Superconducting tunnel elements for detection purposes have existed for approximately 25 years. Cooper pair tunnels and lone-electron tunnels (also called quasi-particle tunnels) are used in radio-astronomy for the detection of electromagnetic waves. SQUID-systems employing superconducting tunnel elements are used in medicine and research for the precision measuring of electromagnetic fields.
Superconducting tunnel elements detect electromagnetic radiation in two different modes: as a wide-band detector, and as a frequency-selective detector. As a general rule, an operating limit for tunnel elements (in terms of small wavelengths) is determined by the energy gap in the excitation spectrum of the superconductor material. The energy gap in turn is a function of the temperature, and theoretically reaches its maximum at T=O. This means that with lower temperatures, the short-wave operating limit is shifted continuously farther in the direction of smaller wavelengths.
Because of the dependency of the response time and the switching time on the energy gap, these times are also continuously reduced with a decreasing temperature. Furthermore, as the temperature decreases, the noise temperature is reduced and sensitivity is increased. The value of the energy gap may be controlled, for example, by means of a current loop, between the values of the material-specific and temperature-dependent maximum down to zero. Therefore any spectrum can be adjusted starting from the short-wave limit as shown from German Patent Document DE-OS 18 03 953.
The basic structure of a tunnel element consists of two or more layers of superconductor material separated by a barrier. Two different tunnel effects may be utilized for the detection of electromagnetic radiation, with the thickness of the barrier determining the type of the effect, as discussed hereinafter. In the case of barrier thicknesses of more than 1.5 nm to approximately 3 nm, only lone electrons can still tunnel. Cooper pairs, on the other hand, tunnel at thicknesses of around 1 to 1.5 nm.
Much development work has been done recently in the field of superconducting tunnel elements and their applications, as evidenced by the following:
Series connections with more than 1,000 Josephson tunnel elements have been manufactured and studied. (J. Niemeyer and J. H. Hinken, Mikrowellen Maqazin, Vol. 13, No. 2, 1987, Page 118).
European Patent Document EP 329 507 A1, discloses a superconducting tunnel element in which a plurality of superconducting tunnel layers and insulating layers are stacked on one another. The layers have thicknesses of only a few atomic layers (Column 3, Line 58) and may be applied by means of molecular beam epitaxy (Column 4, Line 20). Electrical connections (for example, for control voltages) within the individual layers of the stack are not disclosed.
The U.S. Journal J. App. Phys. 59(11), 1986, Page 3807, discloses an optical detector with a superconducting oxide layer.
From Patent Abstracts of Japan of the Japanese Patent Document JP 63-248187 (A) a stack of superconducting and normally conducting thin films is known.
From Patent Abstracts of Japan of the Japanese Patent Documents JP 02-5580 (A) and JP 2-114576 (A), superconducting optical detectors are known, in which several superconducting layers and insulating layers are stacked on one another.
Patent Abstracts of Japan, Japanese Patent Documents JP 01-53584 (A) and JP 01-53585 (A), disclose analog/digital converters which consist of stacks of superconducting layers of different sizes which are separated by insulating layers.
U.S. Pat. No. 4,837,604, discloses a superconducting switch with a stack of superconducting layers in which the control current is fed to a layer situated in the stack.
In German Patent Document DE 40 10 489 A1, a superconducting tunnel element is used to provide a sensor and a transmitter in which the frequency range can be adjusted. This tunnel element also detects electromagnetic radiation only in the long-wave spectral region which lies above a short-wave limit which corresponds energetically to the respective value of the energy gap. As noted previously, the value of the energy gap, in the case of the tunnel elements, can be controlled between the values of the material-specific and temperature-dependent maximum to zero.